The present invention relates to a drive circuit for a thin film EL display panel such as an AC driven capacitive flat matrix display panel.
The construction of a double insulation (or three-layered) thin film EL display panel is described below with reference to FIG. 10.
Strips of transparent electrode (2) composed of In.sub.2 O.sub.3 are put in parallel to one another on a glass substrate (1). Then a dielectric layer (3) composed of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 or Al.sub.2 O.sub.3, an EL layer (4) composed of ZnS doped in activating agent such as Mn, and another dielectric layer (3') composed of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 or Al.sub.2 O.sub.3 each with thickness between 500 and 10,000.ANG. are deposited in turn, by a thin film technology such as evaporation or sputtering, on the transparent electrodes (2) to form the three-layered construction. Finally, strips of counter electrode (5) composed of Al.sub.2 O.sub.3 are provided, at right angle to the transparent electrode (2), on the three-layered construction.
The thin film EL element thus obtained is considered as a capacitive element in terms of circuit equivalence because the EL layer (4), clamped between the two dielectric layers (3) and (3'), is placed between the electrodes. As obvious from the voltage-to-luminance characteristic shown in FIG. 11, the thin film EL element is driven by a relatively large voltage of the order of 200 V.
Conventionally, the thin film EL display panel with such construction is driven by a field reversal drive unit which is equipped with an N-ch MOS driver and a P-ch MOS driver as scanning side electrode drive circuits and reverses the polarity for each field (for each line sequential drive of a field). Since the EL element construction is not symmetrical with respect to the emitting layer, however, application of write voltage with its polarity reversed for each field will cause luminous intensity variation in a picture element between fields, thus resulting in flickering pictures.
In the U.S. patent application Ser. No. 737,068 by Harada et al. (The British counterpart is Application No. 8513058 and the counterpart in West Germany is Application No. P3518596.1), the applicant has proposed a drive circuit which employs an N-ch high withstanding MOS driver and a P-ch high withstanding MOS driver for field reversal drive of a scanning, side electrode. This circuit reverses the polarity of the write waveform applied to a picture element for each scanning line, thereby eliminating luminous intensity irregularity caused by the polarity inversion of the voltage applied to the panel, and minimizing flickers in a picture.
With the proposed drive circuit, the scanning period of a scanning line involves three different drive periods; precharge period (10.mu.s), discharge/pull-up charge period (10.mu.s) and write drive period (30.mu.s). This means at least 50.mu.s is required for sufficiently high luminance of a scanning line. Accordingly, it is necessary to use lower frame frequency as the number of scanning side electrodes increases, which further causes a picture of a poor quality with flicker and low luminance.
In addition, according to the proposed drive circuit, the charged electrodes are discharged and the potential of the electrodes is pulled up in the reverse direction. This drive method involves large power consumption in modulation.